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Crude oil tankers, however, aren't explosive at all.

Well, if they are empty (actually a little oil remains in the tanks), the tanks could be filled with fumes (unless some kind of ventilation is running), which could make them quite explosive.
 
Well, if they are empty (actually a little oil remains in the tanks), the tanks could be filled with fumes (unless some kind of ventilation is running), which could make them quite explosive.
That is true.

When fully loaded, tankers are very resistant to even anti-ship missiles, which was demonstrated during the Iran-Iraq-US "tanker wars".